[60]fullerene and three [60]fullerene derivatives in membrane model environments

Aggregation of C-60 and of three C-60 derivatives in phosphatidylcholine liposome model membranes was studied via photophysical investigations. The triplet properties of these fullerenes, including the absorption spectrum between 400 and 900 nm, molar absorption coefficient, quantum yield of formation and/or quantum yield for the photosensitised production of O-1(2) were determined in benzene solution and in liposomes. One of the derivatives was a cyclopropyl-fused hexa-addended C-60 containing as addend C(CO2C18H37)(2) (1). The other two derivatives were cyclohexyl-fused C-60 with leucine (2) or hydrophilic acidic (3) mono-addend functionalisation. The synthesis of derivative 2, by thermal extrusion of sulfur dioxide from the corresponding sulfone in the presence of C-60, is described. [60]Fullerene and derivative 1 show no T-T absorption in liposomes, although they do so in benzene solution (with a low triplet quantum yield for hexa-addended 1). The absence of triplet-triplet absorption in liposomes is indicative of fullerene aggregation. By contrast, the amphiphilic mono-addend derivatives 2 and 3 present appreciable T-T absorption in liposomes, although less marked than in benzene solution. Mono-addend functionalisation therefore appears to be an interesting way to prevent or diminish fullerene aggregation. Moreover, in this type of derivatisation most of the photophysical properties of pristine [60]fullerene are retained.